The operating conditions of existing lining plate for use with tube mill of cement industry is middle and low pressure, and middle and low impact force. The failure frequency of reciprocation force is less than 10 to the power of 7, belonging to low cycle fatigue. The wear type is abrading-type abrasive wear with middle and low stress. This kind of operating condition is collectively called “Small Energy & Repeated Impact” condition.
So far, the lining plates for use with tube mill of cement industry home and abroad are wear-resistant cast steel lining plates manufactured with such steelmaking equipments as electric arc furnace and main frequency furnace, through the steps of ingredient preparation and smelting as well as making sand mold or metal mold for the lining plates, then the steps of casting, removing from mold and heat treatment. Its mechanical and physical performance index is as follows: the Rockwell Hardness HRC is about 55-60, the impact value α is 4˜8 J/cm2, and the metallurgical structure is a Martensite, such as a carbide. The submicroscopic pattern of the carbide is a continuous network pattern, functioning as microcracks for damage within the steel materials. The mechanical and physical performances can be greatly improved only under the condition that cracks occur to the network carbide. This is also a long-term goal to strive for scientific and technical workers, but so far with little success. This is one of the key problems of technologies resulting in a short service life of wear-resistant steel material.
As a result, existing tubes mill of cement industry all use large thickness of lining plates, to ensure an ideal maintenance cycle and an appropriate service life.
For example, the lining plate thickness of tube mill is generally more than 65 mm, and the total weight of lining device is up to 105 T. With a large thickness and a great weight, the fixing of milling tube body can be achieved only by bolt connections, thus the total drilling holes of the tube body can be up to 1632, seriously weakening the stiffness and strength of the tube body. Depending on the demands of economical maintenance cycle (service life), the thickness of lining plate is about 65 mm, where the size of each lining plate is approximately 250×350×60 mm, so the weight of each lining plate is about 44 kg. In such conditions, proposals of fixing with other ways are suggested, for example, a test has been performed in 1960s, where a fixing method for inlaying and sticking by use of adhesives is used. However, a slight mistake in installing may lead to a collapse, thereby causing serious human injuries, or a collapse of lining plate in the milling tube when operating will result in serious facility accidents.
It cannot realize a weight reduction by decreasing the size, unless the thickness of lining plate is reduced. Along with the upsizing of tube mill, the size of lining plate is increasing, even more unsuitable for sticking with adhesive.
A major breakthrough has never happen to the service life of wear-resistant cast steel lining plate of tube mill, always within 7000-10000 hours. The total amount of lining plates for use with tube mills of cement industry in Chinese mainland is up to more than 300,000 tons. Therefore, significantly extending the service life of lining plate and thus saving steel consumption has a great economic significance both for energy conservation and environmental protection.
Thus, to reduce the thickness and weight of lining plate, increase the filling amount of grinding body, and greatly the production of mill tube; to renovate the fixing way of lining plate, realize that the tube body has no or few drilling holes thus enhancing the rigidity and strength of tube body; and to guarantee the reliable operation of equipment and so on, are the achievements that scientific and technological works have always dream, but with little success even to this day.
Along with the development of grinding technology, it can significantly save energy by use of pre-grinding technology in 1980s, which was gradually familiar to people. In existing cement industry, it is a general trend to gradually install such pre-grinding equipments as extruding press, vertical mill and short mill to the tube mill. In this way, the granularity of grinding materials has been lowered from 25-30 mm to less than 1 mm, even gradually to 150-200 m2/kg of specific surface. Therefore, fundamental changes have occurred to the grinding mechanical property of tube mill. In one word, the impact force of the lining plate received in the tube mill is significantly decreased.
Consequently, the requirements on the material of lining plate of tube mill are no longer limited to followings: in order to withstand the impact force without cracking, both a good impact ductility a value (20-30 J/cm2) and a certain cross section are needed, and the cross section is the thickness of lining plate.
The impact value α and Rockwell Hardness HRC of the wear-resistance steel material cannot be achieved at the same time. To increase the impact value α, the Rockwell Hardness HRC must be sacrificed; conversely, to improve the Rockwell Hardness HRC, the impact value α must be lowered. When the impact value α 20˜30 J/cm2, the Rockwell Hardness HRC can be 40-45 at most, thus it must have a lining plate thick enough to meet the need of economic maintenance cycle (service life).
With the pre-grinding process, the tube mill performs grinding mainly by the grinding function. It is better to have a higher Rockwell Hardness HRC (preferably 55-60) so as to withstand grinding wear and realize an enough service life, as long as the material of lining plate has an appropriate impact value α (20˜30 J/cm2). HRC can be improved only with low α value. In fact, the development of existing lining plate technology is greatly lagging behind, and it provides the present invention with space and opportunities for development.